This invention relates in general to corrosion inhibition of steel, and more particularly, to a method of treating oxidized or rusty surfaces of carbon and low alloy steels for reliable acceptance thereon of a paint or protective coating. Heretofore, iron, steel, and low alloy steels, hereinafter referred to as steel, contrary to stainless steels, aluminum, magnesium, zinc, copper, nickel, chromium, lead, silver, gold, etc., exhibit a high degree of corrosibility in atmospheric environments, and they require expensive and timeconsuming surface preparation procedures for reliable acceptance of corrosion protective coatings and paints thereon, e.g. sandblasting, wire brushing, grinding, acid cleaning, etc. to remove the oxidized products of corrosion. It is well known that oxidized steel or rust has relatively little tensile strength to the unoxidized steel substrate so when a conventional paint primer containing an organic binder or vehicle is applied, the primer on curing customarily shrinks 50 percent or more in all directions thereby stressing the adhesion of the oxidized metal to the substrate and thus conducing to early disbondment of the primer together with any subsequently applied topcoats. The relatively high tensile strength of the organic film, or as otherwise stated, its contractibility upon curing, presents a force disruptive of the adherence of the oxidized steel to the substrate. Another problem unsatisfactorily solved to the present time is caused by the fact that the primers heretofore used are slow drying which results from the organic character of the constituent vehicle. Among such vehicles are alkyd resins, oleo resins, linseed oil, fish oils, and vegetable oils, all of which cure or harden through relatively slow oxidizing reactions. Such primers, whether they be used upon oxidized or unoxidized metal surfaces have exhibited a lack of adequate porosity to permit the migration of soluble corrosion inhibiting ions, such as chromates, phosphates, molybdates, and the like to the metal substrate through the primer film, comprehending a migration through the natural pores of the oxidized steel to the unoxidized steel where such ions can react to inhibit further corrosion. Furthermore, when oxidized steel is primed with an organic primer, the resulting film dimensionally shrinks on curing within the interstices of the oxidized steel film thereby opening up microscopic stress cracks which conduce osmotic migration of water under the primer and the paint coatings resulting in blistering and disbondment of the topcoat.
It has been customary to the present time to use phosphates as corrosion inhibiting compounds in so-called "wash primers" in pretreatment of steel before priming. However, in such cases the oxidizing reaction with the metal substrate is prior to the application of the topcoat and since such compounds are acid in nature, the application of the same is hazardous to personnel and thereby require closely controlled conditions for application.
Therefore, it is an object of the present invention to provide a method for treating oxidized steel surfaces wherein a solution is utilized which upon curing is inorganic and does not contract upon hardening so as to stress the adhesion of the steel oxides to the substrate and thereby conduce to ultimate disbondment.
Another object of the present invention is to provide a method of the character stated wherein the cured solution dimensionally fills the pores of the oxidized steel film and thereby improving and enhancing the shear strength, compressive strength, and curing strength of the resultant film.
It is another object of the present invention to provide a method of the character stated wherein the solution utilized contains a liquid vehicle or liquid binder system exhibiting comparable rust penetrating qualities as do organic primers, but which cure and harden by solvent evaporation and hydrolyzing reactions with environmental moisture; and thus with the present invention moist environments are conducive to fast curing time and fast recoat time.
It is a further object of the present invention to provide a method of the character stated which comprises treating an oxidized metal surface with a solution having an organic vehicle which, on curing becomes inorganic, and develops shrinkage cracks, pores, and small interstices within the resultant film, which latter are continuous in nature and provide paths for travel of soluble corrosion inhibiting ions, through the oxidized steel to the unoxidized steel substrate, thereby controlling further undesired oxidation.
It is a still further object of the present invention to provide a method of the character stated whereby osmotic water blistering of the topcoat is inhibited.
It is a still further object of the present invention to provide a method of the character stated wherein the applied solution may contain neutral metal phosphate salts which are nonhazardous to the applying personnel and which react in an oxidizing manner with the anodic metal substrate, causing formation of insoluble metal phosphates deposited in the cracks, pores, interstices of the cured film to prevent corrosion undercutting of applied topcoats.
It is another object of the present invention to provide a method of the character stated which eliminates sandblasting of oxidized steel surfaces prior to application of the solution which eliminates "lost" sandblasting efforts due to unexpected rainfall; which accelerates job completion through more efficient use of labor; which permits treatment (may also be called priming) at low temperatures and under moist conditions; which permits of application of the solution over damp surfaces; and which permits first topcoat applications within approximately five minutes subsequent to the application of the solution thereby reducing rigging time for moving paint stations, scaffolding, and like support structures; and which permits waterblasting and the elimination of objectionable generation of dust.